1. Field of the Invention
The present invention relates to a magneto-optic recording medium onto and from which information is recorded and reproduced by means of a laser beam. More particularly, the invention relates to a high-density magneto-optic recording medium and an information reproducing method using the recording medium.
2. Description of the Related Art
Attention is being focused on a magneto-optic recording medium for use in an erasable high-density recording method in which information is recorded by forming magnetic domains in a magnetic thin layer using the thermal energy of a semiconductor laser, and the information is read from the magnetic domains by use of the magneto-optic effect. There is an increasing demand for increasing the recording density of this type of recording medium in order to obtain a larger capacity.
The linear recording density of an optical disk, such as the above-described type of magneto-optic recording medium, significantly depends on the laser wavelength .lambda. of a reproducing optical system and the numerical aperture (NA) of an object lens. In other words, the reproducing optical wavelength .lambda. and the numerical aperture NA of the object lens determine the size of a beam waist. Accordingly, the smallest possible mark to be reproduced is as low as .lambda./2NA. On the other hand, the track density, as well as the smallest mark, is primarily restricted by crosstalk between adjacent tracks, and thus relies on the spot size of a reproducing beam. Accordingly, it is necessary to shorten the laser wavelength .lambda. of a reproducing optical system or to increase the numerical aperture NA of an object lens to increase the density of a conventional optical disk. It is not easy, however, to shorten the laser wavelength because of the efficiency and heat generation of devices. Also, an increase in the numerical aperture NA of the object lens will make the working of a lens complicated and also cause mechanical problems, for example, the proper distance between the lens and the disk cannot be sufficiently ensured, causing the two elements to collide with each other. In view of this background, a technique for improving recording density is under development by enhancing the construction of a recording medium or improving the method of reading information.
Meanwhile, the present inventors have attempted to develop a magneto-optic recording medium which can achieve magnetic super-resolution without applying a reproducing magnetic field. They have also attempted to develop an information reproducing method using this medium.
For example, the present inventors have suggested a magneto-optic recording medium constructed as shown in FIG. 1, as disclosed in Japanese Patent Laid-Open No. 6-124500. This publication discloses a super-resolution technique for attaining a recording density higher than the optical resolving power of the reproducing light. FIG. 1A is a sectional view of an optical disk representing a super-resolution technique by way of example. The arrows shown in the magnetic layers indicate the directions of the sub-lattice magnetization of, for example, an iron-group element contained in the magnetic layers. A recording layer 42 is formed of a TbFeCo or DyFeCo film exhibiting a high level of perpendicular magnetic anisotropic characteristics. The recording information is recorded and stored in the form of magnetic domains in the upward direction or the downward direction relative to the surface of the film. A reproducing layer 41 remains in the form of an in-plane magnetic film at room temperature, but is transformed into a perpendicular magnetic film when the temperature is raised to reach Tm (described below). For reproducing the information, light is applied to a side of a substrate 20 of the disk formed of the medium constructed as described above. The resulting temperature gradient at the center of the data track is indicated as shown in FIG. 1C. As this temperature gradient is viewed from the substrate 20, a Tm isothermal line can be seen within a light spot 22, as shown in FIG. 1B. As described above, a region of the reproducing layer at a temperature lower than Tm remains in the form of the in-plane magnetic film, which does not contribute to a high level of the Kerr effect (a front mask 24 is formed). Accordingly, recording magnetic domains stored in the recording layer 42 are masked and unseen. On the other hand, a region of the reproducing layer 41 at a temperature of Tm or higher is transformed into the state of a perpendicular magnetic film, and the magnetization in such a region is oriented in the same direction as the recorded information due to an exchange interaction coupling force with the recording layer 42. As a result, recording magnetic domains stored in the recording layer 42 are transferred only to an aperture 23 smaller than the light spot 42, thereby realizing super-resolution.
In this known super-resolution reproducing method, the track density, as well as the linear recording density, can be improved with the arrangement in such a manner that the front mask 24 located in the low-temperature region extends along an adjacent track.
However, the super-resolution magneto-optic recording medium constructed of double layers using the in-plane magnetic film presents the following problems.
The magnetic information stored in the recording layer can be sufficiently masked when in-plane anisotropic characteristics in the reproducing layer are increased to a higher level at room temperature. It is difficult, however, to completely transform the reproducing layer to the state of a perpendicular magnetic film at a reproducing temperature. Conversely, when in-plane anisotropic characteristics are decreased to a lower level at room temperature, the reproducing layer is completely transformed into the state of a perpendicular magnetic film at a reproducing temperature. At room temperature, an interfacial magnetic domain wall generated between the reproducing layer and the recording layer is generally biased toward the reproducing layer, as illustrated in FIG. 2. More specifically, perpendicular magnetic components representing the magnetic information stored in the recording layer are disadvantageously generated in a portion of the reproducing layer adjacent to the recording layer. It is thus difficult to completely mask the magnetic information stored in the recording layer with the reproducing layer. As a consequence, when the recording mark or the track width is decreased in the above type of conventional recording medium, good reproducing signals cannot be easily obtained.
Also, in this recording medium, it is necessary that the reproducing layer be formed thick enough to mask the information in the form of the magnetic domains stored in the recording layer in order to obtain a high degree of S/N (signal-to-noise) ratio (C/N ratio). More specifically, as disclosed in Japanese Patent Laid-Open No. 4-25593, the reproducing layer having a thickness of 150 .ANG. or smaller produces an adverse influence on the layer just below the reproducing layer by 25% or greater, which makes it impossible to achieve super-resolution reproducing. Accordingly, it is necessary that the thickness of the reproducing layer be in a range of from 200 .ANG. to 300 .ANG. or higher in order to obtain signals having a high degree of S/N ratio required for practical use. In the manner described above, in this recording medium, the thickness of the overall magnetic layers including the reproducing layer cannot be reduced, since it is necessary to mask the information in the form of magnetic domains stored in the recording layer.
There is an increasing demand for enhancing recording density by increasing the linear velocity of a magneto-optic recording medium. However, it is difficult to meet this demand for the recording medium because of the following reason. There is a limitation for the output of the optical power, such as a semiconductor laser, although a large amount of optical power is required for recording information onto a medium provided with a thick magnetic layer having a large heat capacity. This also means that a reflective layer cannot be disposed to form the medium in the enhanced structure, and thus, the C/N ratio cannot be increased. Further, metal formed of a rare-earth element, which is costly, is generally used for magnetic layers, which further increases the costs of the overall recording medium provided with thick magnetic layers. Because of these reasons, it is difficult to provide an inexpensive magneto-optic recording medium in which the higher density by means of super-resolution can be achieved simultaneously with faster recording.